It is not yet known whether an antigen-specific event or a more global defect in immune regulation allows the initiation of insulitis in insulin- dependent diabetes mellitus (IDDM). This grant intends to explore two competing hypothesis concerning the "initiation" of auto-immune insulitis. The first, more traditional paradigm: initiation of type I diabetes in NOD mice (and in man) is associated with the major histocompatibility complex (MHC) class II gene products due to presentation of "diabetogenic" islet beta cell antigens to T cells. The contrasting novel hypothesis suggests that autoimmunity in the NOD mouse is, in part, due to "inefficient" I- A/g7 thymic editing (positive and negative selection) leading to a repertoire of CD4 T cells in NOD mice which have high affinity T cell receptor (TCR) regulation of self-peptides. These two hypothesis will be tested by analyzing the earliest islet-infiltrating NOD T cells. T cells will be examined directly from the early insulitic lesions, as well as after adoptive transfer of "early" NOD islet-infiltrating T cells into NOD Rag-2 knockout mice. Hybridomas and T cell clones derived from islet infiltrating T cells will be used to screen a beta cell-enriched expression library. An additional set of experiments will study thymic editing (positive and negative selection) in NOD mice which may lead to the escape of high affinity self-reactive T cells and resultant widespread autoimmunity. The final set of experiments, using cell surface CD4 expression (which increases dramatically on activated T cells), FACS sorting, limiting dilution analyses and T cell cloning, should allow characterization of islet antigen-specific T cells. This grant proposes to study (1) antigens recognized by islet-infiltrating T cells, (2) potential defects in thymic education involved in the pathophysiology of NOD disease and (3) the pathophysiology of inflammatory insulitis in NOD mice. It is proposed that such studies may provide an understanding of components of the pathophysiology of human IDDM.